Accompanying the progress of technology, optical scanners have already become a popular computer peripheral device. An image capturing method for an optical scanner mainly uses a light emitting device to project light onto a document. The light is guided to a light sensing element through a lens set after being reflected by the document, and the light sensing element can check and measure light with different strength reflected from different areas on the document. The reflected light wave is then converted to digital data. Thereafter, scanning software is used to read the data and reassemble it into a computer image file.
Please refer to FIGS. 1 and 2. An ideal light emitting device should have a uniform brightness distribution, but the current scanner modules mostly use a charge coupled device (CCD) as a light sensing element A1 and a fluorescent tube A2 as a light emitting device so as to utilize the fluorescent tube A2 to illuminate and scan a document A5 placed on a platform A3, and reflect light emitted from the fluorescent tube A2 to the document A5 through a light reflecting sheet A6. The result is that the light sensing element receives light unevenly, causing the light received to be brighter at a middle part and dimmer at the parts of two sides, influencing the scanning quality after reflectors A4 reflect light to CCD A1, because the brilliance of the fluorescent tube A2 at the middle part thereof is much higher than the brilliance at the two ends thereof, or it is limited by a characteristic of an optical lens.
Due to the increase in environmental awareness, a fluorescent tube such as external electrode fluorescent (EEFL) or cold cathode fluorescent (CCFL) used in the scanner module is gradually being replaced by other light sources, because it contains mercury or other harmful substances; a light emitting diode (LED) is currently one of the best substitutes. Accompanying the popularity of LEDs, a LED light strip structure replacing the fluorescent tube has emerged; it is formed as a light source device similar to a fluorescent tube by connecting LED pellets to one another in series in an equidistant arrangement on a long-strip type substrate. However, the illumination brightness of the entire body has the same characteristic problem as the fluorescent tube, namely, there is a significant drop between the brightness of a middle part thereof and the brightness of the two ends thereof. To solve this problem, the brightness of the two ends can always be increased by means of firmware (F/W) compensation at the end of the process, to allow the brightness to be as uniform as possible to achieve improved image brightness.
However, the manner mentioned above is unable to improve a signal/noise ratio (S/N ratio); the noise is also amplified after the brightness of the two ends is increased by means of the firmware (F/W), causing an image to be distorted.